Plasma surface treatment is well known in particular in the glassmaking field. It consists of generating a plasma between at least two electrodes and injecting precursor products into this plasma that result in the occurrence, by reaction and/or ionisation, of reagents that act on the surfaces to be treated.
PECVD (plasma-enhanced chemical vapour deposition) may be applied using any plasma: cold plasmas (not in equilibrium) or thermal plasmas (in equilibrium). Cold plasmas are generally preferred. The active species (electrons, ions, metastables, etc.) of the plasma typically possess energies of a few eV and may thus cause dissociation or activation of chemical precursors. To maintain the non-equilibrium plasma, it is often necessary to work at reduced pressure. Most known PECVD techniques therefore use low-pressure plasmas.
However, to apply this process for industrial purposes, it is necessary to minimize the costs. There is therefore a growing interest on the part of industrial manufacturers to put aside low-pressure plasma technologies and go for plasma technologies operating within a pressure range close to atmospheric pressure.
Various plasma types are known in plasma technologies: “Glow discharge plasma” or homogeneous plasma allows deposition of very homogeneous thin-film coatings and requires a relatively low energy level. However, it is lengthy and must be limited within a restricted frequency field to remain stable. It also allows a more restricted variety of thin-film species.
Raising the energy level of plasma may cause the outbreak of electric arcs. Placing a dielectric plate between the electrodes allows obtaining an intermediate state between glow discharge and electric arcs, named “filamentary” state. Filaments are intrinsically unstable but carry a high energy level, allowing a reduction of the time of processing and thus to accelerate the speed of the substrate. On the other side, owing to their random production, a paradoxically homogeneous deposition rate of materials is obtained, a very high number of micro-discharges being produced during a cycle on a given area. The use of a dielectric barrier discharge (DBD) has the advantage, over the other plasma processes, of operating at atmospheric pressure, and of allowing continuous treatment on large areas. Furthermore the energy supplied by a filamentary plasma may be finely modulated, thereby making it possible for films having a large variety of compositions to be deposited.
A process for depositing layers onto a substrate by dielectric barrier discharge is known from document EP 2145978. The described process includes in particular the insertion or passage of a substrate in a reaction chamber, in which an electrode and a counter-electrode are positioned. A dielectric barrier is placed between this electrode and this counter-electrode. A high-frequency electric voltage that causes the generation of a plasma is generated between the electrode and the counter-electrode. A gaseous composition is fed into the reaction chamber that upon contact with the plasma reacts with the surface of the substrate.
The electrodes are exposed to very severe working conditions: the high temperature of the plasma, the high reactivity of the injected and/or generated products, the voltage, current and frequency conditions for generating electrostatic forces and the occurrence of arcs at the surface of the electrode can cause localised breakdowns or can simply destroy the electrode. These problems are all the more evident in the case of electrodes placed in production lines with surfaces of large dimensions such as volumes of flat glass. A known method for reducing these problems is to glue the electrically insulating layer onto the face of the electrode facing the surface to be treated that is intended to be exposed to high voltage, e.g. according to WO 2011/134978.
However, in known installations we have observed concerns of contamination (e.g. soiling, clogging) of the high-voltage electrode and the dielectric in its vicinity that can result in a limited rate of deposition of the layer on the substrate as well as edge effects that can cause hot arcs that can in turn create irregularities in the layer and/or fractures of the substrate when it is made of glass and/or can damage the electrodes.